The starting four lines is enough to see how much memory is it actually using, The next details if useful, if you want to pin point which library or usually plugin lib, which is eating memory , and other analysis.

here u see that actually, over 90.5 MB ( that is 21956 kb private clean + 76272 kb private dirty ) Is used by firefox , as reserved private region, exclusively. and only over 6 mb is occupied by shared libs, used by many processes along with Firefox.

actually, Though this is a better solution, it is not ideal. See for example, All this shared libraries is used only by this process, or else just by 2 or 3 processes, than this one is not ideal.